1. Field of the Invention
The present invention relates to a gas turbine moving blade steam cooling system, and more specifically to a structure thereof which is able to prevent strength reduction of blade root portion and also to prevent steam leakage.
2. Description of the Related Art
FIG. 8 is a cross sectional view of a prior art gas turbine interior and shows flows of cooling air in a moving blade portion. In FIG. 8, numeral 50 designates a stationary blade, numeral 51 designates an outer shroud and numeral 52 designates an inner shroud. Numeral 60 designates a moving blade, which is fixed to a blade root portion 62 of a turbine disc 61 and rotates between stationary blades 50.
In the prior art gas turbine so constructed by the stationary blade 50 and the moving blade 60, the moving blade 60 is cooled by air which is a part of rotor cooling air. That is, there is bored a radial hole 65 in the blade root portion 62 and the rotor cooling air 100 is introduced into each disc cavity 64 to be further introduced into a lower portion of a platform 63 via the radial hole 65 and then is supplied into the moving blade 60.
FIG. 9 is a cross sectional view of a moving blade portion and a stationary blade portion of the gas turbine shown in FIG. 8. In FIG. 9, numeral 50 designates a stationary blade, which has an outer shroud 51 and an inner shroud 52 as well as an air pipe 53 extending in a blade height direction and passing through the blade interior. Seal air 110 is fed through the air pipe from the outer shroud 51 side into a cavity 54 so that pressure in the cavity 54 is made higher than that in a combustion gas passage and the seal air 110 further flows through a hole 57 and is partially discharged from a passage 56 so that a high temperature gas is prevented from coming therein. Numeral 55 designates a labyrinth seal, which is also for sealing the high temperature gas.
As for the cooling air for the moving blade 60, the mentioned rotor cooling air 100 is introduced into the disc cavity 64 to be further introduced into a shank portion 66 of a lower portion of the platform 63 via a radial hole 65 which passes through the interior of a rotor disc blade root portion 62 and then is supplied into a cooling air passage in the moving blade 60. Further, in place of using a portion of the rotor cooling air, it takes place also that air from a compressor is cooled by a cooler and is introduced into a disc cavity 64.
As mentioned above, the conventional process of cooling the gas turbine blades is air cooling and, especially for the moving blades, a portion of the rotor cooling air is introduced so as to be used for cooling thereof. In recent years, a steam cooling method is being developed in order to effect steam cooling of the rotor system, and it is imperative to employ such a structure that steam leakage is prevented sufficiently and the blade root portion, in which steam passages are provided, may adequately withstand thermal stress.
Further, in the case of air cooling, there occurs a lot of air leakage when the cooling air enters the moving blade from the disc which results in a loss of cooling air, while, in the case of steam cooling of the moving blade, there is no such loss of cooling air but if the steam escapes, a large amount of steam on the boiler side is lost which affects the performance greatly.
Also, in the moving blade of the air cooling method, there occurs stress concentration at a through hole portion of the radial hole between the blade root portion and a blade base portion so as to be affected by thermal stress. Hence, in order to employ steam cooling, it is necessary to consider a structure which avoids the stress concentration.